There are mechanical engineers out there who can literally model objects nearly as fast and they can 'think' about the layout of said object. If you look at the complexity of, say, a CAD model from a real, highly complex aluminum casting section of an automotive subframe, or the living-room-sized cross-fuselage spar forging of a fighter aircraft, with hundreds of ribs and fillets and features- and compare that to the simple model you are trying to make in OpenSCAD, you should quickly realize the parallels in difficulty you are trying to express (similar to the person without knowledge of C++ or Python watching someone be able to build applications by typing code from their fingertips as if they already knew what to type...)
You are struggling for a few reasons- 1., it is a knowledge hurdle of an entire field you are trying to surmount- again, go watch someone actually model a real, complex part and watch the speed, they can do so in a tool like Solidworks, CATIA, NX, etc... at a rate that is far different because they have experience that it can honestly take even good people years to accumulate - and 2. they are using professional tools - you mention OpenSCAD, like it is CAD, but it isn't. It is programmatic mesh generation, and it turns out that programmatically typing out how to generate complex things is much more difficult than a combination of a graphical GUI and graph-based generator that all big CAD programs figured out starting in the 1980s. If those tools you use were really the best way to make complex models 'paramaterized', then why do we design our fighter jets, Formula 1 cars, or Space X rockets in Dassault's CATIA or Siemen's NX ?
You want a LLM to take a sketch to your CAD, but what I'm saying is, there are people out there that can skip the sketch and build the CAD as fast as you can likely hand draw the first sketch, and these are skills you can actually learn, but you may just be using the wrong tools and have not had the practice necessary.